Dear Brie,
Over a year ago, in order to better support fluency coding, we introduced a distinction between the use of the ampersand to mark non-words and its use to mark filler words. For filler words, we added the hyphen mark as in &-uh and &-um. It is clear that &-uh is always a filler. However, the sound "oh" is often what we would call a "co" or communicator, as in "Oh, I didn't realize that." If it is a real word communicator, then it is just oh. If it is a filler, which is rather rare, then it is &-oh. We still use the basic & mark as in &gok for things that are just non-words. And there are a two more uses of the ampersand, including &=text as in &=laughs for laughing and &* for interposed words (section 9.10.2 of the manual).
-- Brian MacWhinney